An envelope pillow is probably the easiest and fastest way to make a pillow cover. If you don't have a lot of sewing experience or you don't want to bother dealing with a zipper this is a great option.
Materials
An 18"x18" piece of fabric that you will use as your "front". In this case I used my punch needle piece.
Two pieces of fabric that you will use as your "back" that are 18" in width and 14" high. These could also be 12" in height depending on the fill of your pillow. If your pillow is super fuffly the back flaps might not be long enough to close properly (note: if you are sewing a punch needle piece that is exactly 18" wide you want your back pieces to be 19" wide so you have room for your seam).
Thread to match
Scissors
Pins or sewing clips
Instructions
Take your back pieces and fold over about 1/4" and press. Fold a second time and press.
Sew the hem on each piece.
Lay your front piece, right side facing up.
Lay your back pieces overlapping on top, right side facing down.
Pin all the way around the pillow so the pieces don't move as you sew.
Sew all sides using a 1/4" seam allowance. If you are using a punch needle piece you want to sew as close as you can to the edge.
Turn right side out, you're done!
If you'd like to try this particular punch needle pattern go here.
I used my punch needle piece as a guide and made sure I was right at edge, maybe even a bit further in, so that when I turned it I wouldn't see the light colour of the monk's cloth. If you're using regular fabric it's probably easier to sew looking at the back side.
I got my down filled pillow form from Ikea. Super affordable and great quality. Note that they don't actually have 18" pillow forms, I used a 20" and it fit just fine!
I'm enjoying making multiple covers so I can switch them out depending on the season, hope you do as well!